Network Working Group J. Uberti
Internet-Draft Google
Intended status: Standards Track July 15, 2013
Expires: January 16, 2014
A REST API For Access To TURN Servicesdraft-uberti-behave-turn-rest-00
Abstract
This document describes a proposed standard REST API for obtaining
access to TURN services via ephemeral (i.e. time-limited)
credentials. These credentials are vended by a web service over
HTTP, and then supplied to and checked by a TURN server using the
standard TURN protocol. The usage of ephemeral credentials ensures
that access to the TURN server can be controlled even if the
credentials can be discovered by the user, as is the case in WebRTC
where TURN credentials must be specified in Javascript.
Status of This Memo
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This Internet-Draft will expire on January 16, 2014.
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To address this problem, this document proposes an API that can be
used to retrieve ephemeral TURN credentials from a web service.
These credentials can then be used as long-term credentials with a
standard TURN server with a custom authentication module. For
simplicity, the design has been kept intentionally stateless; the
only interaction needed between the web service and the TURN service
is to share a secret key.
2. HTTP Interactions
To retrieve a new set of credentials, the client makes a HTTP GET
request, specifying TURN as the service to allocate credentials for,
and optionally specifying a user id parameter. The purpose of the
user id parameter is to simplify debugging on the TURN server, as
well as provide the ability to control the number of credentials
handed out for a specific user, if desired. The TURN credentials and
their lifetime are returned as JSON, along with URIs that indicate
how to connect to the server using the TURN protocol.
To avoid the need for state passing between the web service and TURN
server, the returned credentials consist of a TURN username that
encodes all the necessary state (expiry time and application user
id), and a TURN password that is a digest of this state, signed with
the shared secret key.
Since the returned credentials are ephemeral, they will eventually
expire. This does not affect existing TURN allocations, as they are
tied to a specific 5-tuple, but requests to allocate new TURN ports
will fail after the expiry time. This is significant in the case of
an ICE restart, where the client will need to allocate a new set of
candidates, including TURN candidates. To get a new set of ephemeral
credentials, the client can simply re-issue the original HTTP request
with the same parameters, which will return the new credentials in
its JSON response.
To prevent unauthorized use, the HTTP requests can be ACLed by
various means, e.g. IP address (if coming from a server), Origin
header, User-Agent header, login cookie, API key, etc.
2.1. Request
The request includes the following parameters, specified in the URL:
o service: specifies the desired service (turn)
o username: an optional user id to be associated with the
credentials
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o key: if an API key is used for authentication, the API key
Example:
GET /?service=turn&username=mbzrxpgjys
2.2. Response
The response is returned with content-type "application/json", and
consists of a JSON object with the following parameters:
o username: the TURN username to use, which is a colon-delimited
combination of the expiration timestamp and the username parameter
from the request (if specified). The timestamp is intended to be
opaque to the web application, so its format is arbitrary, but for
simplicity, use of UNIX timestamps is recommended.
o password: the TURN password to use; this value is computed from
the a secret key shared with the TURN server and the returned
username value, by performing base64(hmac(secret key, returned
username)). HMAC-SHA1 is one HMAC algorithm that can be used, but
any algorithm that incorporates a shared secret is acceptable, as
long as both the web server and TURN server use the same algorithm
and secret.
o ttl: the duration for which the username and password are valid,
in seconds. A value of one day (86400 seconds) is recommended.
o uris: an array of TURN URIs, in the form specified in
[I-D.petithuguenin-behave-turn-uris]. This is used to indicate
the different addresses and/or protocols that can be used to reach
the TURN server. These URIs SHOULD specify a hostname, IPv4, or
IPv6 address for the TURN server, as well as the port and
transport to use; this avoids the need for a DNS SRV or S-NAPTR
lookup as specified in [RFC5928].
Example:
{
"username" : "12334939:mbzrxpgjys",
"password" : "adfsaflsjfldssia",
"ttl" : 86400,
"uris" : [
"turn:1.2.3.4:9991?transport=udp",
"turn:1.2.3.4:9992?transport=tcp",
"turns:1.2.3.4:443?transport=tcp"
]
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}
3. WebRTC Interactions
The returned JSON is parsed into an RTCIceServer object, and supplied
as part of the RTCConfiguration object that is used when creating a
RTCPeerConnection.
Example:
var iceServer = {
"username": response.username,
"credential": response.password,
"uris": response.uris
};
var config = {"iceServers": [iceServer]};
var pc = new RTCPeerConnection(config);
When the credentials are updated (e.g. because they are about to
expire), a new RTCConfiguration with the updated credentials can be
supplied to the existing RTCPeerConnection via the updateIce method.
This update must not affect existing TURN allocations, because TURN
requires that the username stay constant for an allocation, but the
new credentials will be used for any new allocations.
[TODO: make sure this behavior is specified in the W3C API spec]
4. TURN Interactions4.1. Client
The WebRTC client will perform a standard TURN allocation sequence
using the long-term credentials mechanism specified in [RFC5389],
Section 10.2, using the "username" value from the returned JSON for
its USERNAME attribute, and the "password" value for the password
input to the MESSAGE-INTEGRITY hash.
4.2. Server
The TURN server will process the request using the long-term
credentials mechanism specifed in [RFC5389]. Note that the REALM
value supplied by the server is not meaningful in this context, and
can be set to any valid value.
When processing ALLOCATE requests, the TURN server MUST split the
USERNAME attribute into its timestamp and user id components, and
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verify that the timestamp, which indicates when the credentials
expire, has not yet been reached. If this verification fails, it
SHOULD reject the request with a 401 (Unauthorized) error.
If desired, the TURN server can optionally verify that the parsed
user id value corresponds to a currently valid user of an external
service (e.g. is currently logged in to the web app that is making
use of TURN). This requires proprietary communication between the
TURN server and external service on each ALLOCATE request, and is not
necessary for typical applications. If this external verification
fails, it SHOULD reject the request with a 401 (Unauthorized) error.
For non-ALLOCATE requests, the TURN server merely verifies that the
USERNAME matches the USERNAME that was used in the ALLOCATE (since it
must remain constant).
As in RFC 5766, the TURN server MUST verify the MESSAGE-INTEGRITY
using the password associated with the supplied USERNAME. For the
usage outlined in this document, the password will always be
constructed using the supplied username and the shared secret as
indicated in the "HTTP Interactions" section above. Because the
password is derived from the USERNAME, successful verification of the
MESSAGE-INTEGRITY ensures that the USERNAME (and the expiration time
contained within) is trustworthy.
5. Implementation Notes5.1. Revocation
In the system as described here, revoking specific credentials is not
possible. The assumption is that TURN services are of low enough
value that waiting for the timeout to expire is a valid approach for
dealing with possibly-compromised credentials.
In extreme abuse cases, TURN server blacklists of timestamp+username
values can be supplied by an administrator to stop abuse of specific
credential sets.
5.2. Key Rotation
As indicated in [RFC2104], periodic rotation of the shared secret to
protect against key compromise is RECOMMENDED. To facilitate the
rollover, the TURN server SHOULD be able to validate incoming
MESSAGE-INTEGRITY tokens based on at least 2 shared secrets at any
time.
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Because the USERNAME values in a TURN ALLOCATE request are typically
visible to eavesdroppers, inclusion of an externally identifying user
id, such as a login name, may allow a passive attacker to determine
the identities of the parties in a conversation. To prevent this
problem, use of opaque user id values is recommended.
This mechanism assumes that the clocks of the web server and TURN
server are roughly in sync. Given the expected large TTLs for the
vended credentials, clock skew on the order of seconds to minutes
should not cause an issue. However, if the TURN server's clock was
mistakenly set to a date signficantly in the past, credentials could
be accepted for far longer than their intended lifetime.
7. IANA Considerations
None.
8. Acknowledgements
Harald Alvestrand, Alfred Godoy, and Philipp Hancke provided key
input on the initial design. Dave Cridland, Cullen Jennings, Oleg
Moskalenko, and Matthew Robertson pointed out several errors and
omissions.
9. References9.1. Normative References
[I-D.petithuguenin-behave-turn-uris]
Petit-Huguenin, M., Nandakumar, S., Salgueiro, G., and P.
Jones, "Traversal Using Relays around NAT (TURN) Uniform
Resource Identifiers", draft-petithuguenin-behave-turn-uris-03 (work in progress), January 2013.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
9.2. Informative References
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February
1997.
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